SS.IMX.EstMx.CreAph

SS.IMX.EstMx.CreAph recorded () and expected () distribution in Britain and Ireland (see below)

Ecological and functional relationships

This biotope occurs in the lower estuary where the hydrodynamic regime allows a suitable environment to develop. The presence of a suitable substratum is probably the primary structuring force, rather than the interspecific relationships. Mixed sediment provides a stable substratum for the epifauna such as Crepidula fornicata, Mytilus edulis and ascidians, and soft sediment for the infaunal annelids, crustaceans and bivalves.

Crepidula fornicata competes for nutrients with other suspension feeders, e.g. Mytilus edulis and ascidians. Where Crepidula fornicata is very abundant, trophic competition contributes to the competitive exclusion of commercially valuable species such as Ostrea edulis (Fretter & Graham, 1981; Blanchard, 1997). The faeces and pseudo-faeces produced by Crepidula fornicata contribute to the sediment requirements of the infauna (see 'habitat complexity') and also provide a food source for the deposit feeders, such as Aphelochaeta marioni.

Carcinus maenas is the most important predator in this biotope. It has been shown to significantly reduce the density of Eteone longa, Aphelochaeta marioni, Tubificoides sp. and Corophium volutator (Reise, 1985).

Nephtys hombergi and Eteone longa are active carnivorous annelids that operate at the trophic level below Carcinus maenas (Reise, 1985). They predate the smaller annelids, such as Exogone naidina, and crustaceans, such as Corophium volutator and Cumacea sp.
The amphipod, Corophium volutator, and the infaunal annelid species in this biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows, while high densities of Corophium volutator can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986).

Seasonal and longer term change

Seasonal changes occur in the abundance of the fauna due to seasonal recruitment processes. The early reproductive period of Polydora ciliata often enables the species to be the first to colonize available substrata (Green, 1983). The settling of the first generation in April is followed by the accumulation and active fixing of mud continuously up to a peak during the month of May, when the substrata is covered with the thickest layer of Polydora mud. The following generations do not produce a heavy settlement due to interspecific competition and heavy mortality of the larvae (Daro & Polk, 1973). Variation in abundance is very pronounced in the polychaete Aphelochaeta marioni. For example, in the Wadden Sea, peak abundance occurred in January (71,200 individuals per m²) and minimum abundance occurred in July (22,500 individuals per m²) following maximum spawning activity between May and July (Farke, 1979). However, the spawning period varies according to environmental conditions and so peak abundances will not necessarily occur at the same time each year. For example, Gibbs (1971) reported Aphelochaeta marioni spawning in late autumn in Stonehouse Pool, Plymouth Sound. The adult densities of the bivalveAbra alba typically fluctuate widely from year to year due to variation in recruitment success (Rees & Dare, 1993). The other annelids and ascidians in the biotope are likely to exhibit seasonal variations in abundance, but, again, different areas have local spawning and recruitment characteristics. Crepidula fornicata is a relatively long lived species (8-9 years longevity), suffers low predation and therefore would not be expected to vary greatly in abundance through the year.
One of the key factors affecting benthic habitats is disturbance, which in shallow subtidal habitats increases in winter due to weather conditions. Storms may cause dramatic changes in distribution of macro-infauna by washing out dominant species, opening the sediment to recolonization by adults and/or available spat/larvae (Eagle, 1975; Rees et al., 1976; Hall, 1994) and by reducing success of recruitment by newly settled spat or larvae (see Hall, 1994 for review). For example, during winter gales along the North Wales coast large numbers of Abra alba were cast ashore and over winter survival rate was as low as 7% in the more exposed locations, whilst the survival rates of the polychaetes Eteone longa and Nephtys hombergi were 29% and 22% respectively (Rees et al., 1976). Hayward & Ryland (1995) reported that Crepidula fornicata is sensitive to movement of the substratum during periods of increased wave action and is often found cast ashore following storms. Soft bodied epifauna, such as ascidians, are likely to be very sensitive to storm damage and will probably suffer high mortality during winter storms. Rapid recolonization occurs in summer and therefore abundances are likely to vary considerably due to physical disturbance.

Habitat structure and complexity

The mixed sediment in this biotope is the important structural component, providing the complexity required by the associated community. Epifauna are attached to the cobbles and shell debris and infauna burrow in the soft underlying sediment. Sediment deposition, and therefore the spatial extent of the biotope, is initially dictated by the physiography and underlying geology coupled with the hydrodynamic regime (Elliot et al., 1998). However, once Crepidula fornicata becomes established, it strongly influences the nature of the sediment. Slipper limpets typically attach to a member of the same species, forming chains, which can comprise of up to 12 individuals. In suitable conditions, Crepidula fornicata can reach very high densities; up to 4770 individuals per m2 (de Montaduin & Sauriau, 1999). The resultant shell debris provides a hard substratum for attachment of juvenile Crepidula fornicata, hence perpetuating the population, and also for other epifauna, such as ascidians. Crepidula fornicata also has a major effect on the biotope through the deposition of faeces and pseudofaeces. The deposited sediment can smother other suspension feeders and render the substratum unsuitable for larval settlement (Fretter & Graham, 1981; Blanchard, 1997). In this way, settlement of Crepidula fornicata can initiate a shift away from the oyster beds biotope (IMX.Ost) towards IMX.CreAph. Indeed, this biotope often occurs on relict oyster beds. Conversely, the deposition of faeces and pseudofaeces by Crepidula fornicata can render the substratum more suitable for infauna and deposit feeders (Barnes & Hughes, 1992).

Productivity

Primary production in this biotope comes from benthic microalgae (microphytobenthos e.g. diatoms, flagellates and euglenoides) and water column phytoplankton. Photosynthetic processes may be light limited due to the turbidity of the water (Elliot et al., 1998) and hence primary production is usually low. Large allochthonous inputs of nutrients, sediment and organic matter come from the sea and from discharges of river water containing both naturally derived nutrients and anthropogenic nutrients (e.g. sewage) (Elliot et al., 1998). Secondary productivity in this biotope can therefore be very high and is reflected by the very large abundances obtained by the characterizing species. Crepidula fornicata, for example, can reach densities of 4770 individuals per m² (de Montaduin & Sauriau, 1999) and Aphelochaeta marioni of 108,000 individuals per m² (Gibbs, 1969).

Recruitment processes

Crepidula fornicata is a protandrous hermaphrodite. This means that the animals start their lives as males and then subsequently may change sex and develop into females. Although breeding can occur between February and October, peak activity occurs in May and June when 80-90% of females spawn. Most females spawn twice in a year, apparently after neap tides. Females can lay around 11,000 eggs at a time contained in up to 50 egg capsules (Deslou-Paoli & Heral, 1986). Laboratory experiments by Thain (1984) revealed that, following incubation, approximately 4000 larvae were released per female. Incubation of the eggs takes 2-4 weeks followed by a planktotrophic larval phase lasting 4-5 weeks (Fretter & Graham, 1981; Thouzeau, 1991). Due to the length of the planktonic phase, the potential for dispersal is high. Recruitment will be determined by the local hydrographic regime. For example, in sheltered bays the larvae may be entrapped and small scale eddies (e.g. over obstacles and inconsistencies in the surface of the substratum) may result in the concentration of larvae. The ability of Crepidula fornicata to disperse widely and colonize new areas is demonstrated by its spread through Europe following introduction from North America at the end of the 19th century (Fretter & Graham, 1981; Eno et al., 1997). The spat settle in isolation or on top of an established chain of Crepidula fornicata. Crepidula fornicata needs to be part of a chain in order to breed and therefore would be expected to settle preferentially where high densities of conspecifics already exist. High densities of suspension feeders and surface deposit feeders together with epibenthic predators and physical disturbance may result in high post settlement mortality rate of larvae and juveniles (Olafsson et al., 1994). Males reach sexual maturity 2 months after settlement (Fretter & Graham, 1981). If a male develops directly into a female, sexual maturity may be reached in 10 months (Nelson et al., 1983).
The lifecycle of Aphelochaeta marioni varies according to environmental conditions. In Stonehouse Pool, Plymouth Sound, Aphelochaeta marioni (studied as Tharyx marioni) spawned in October and November (Gibbs, 1971) whereas in the Wadden Sea, Netherlands, spawning occurred from May to July (Farke, 1979). The female spawns puddles of eggs onto the sediment surface adjacent to her burrow. Gibbs (1971) found that the number of eggs laid varied from 24-539 (mean=197) and was correlated with the female's number of genital segments, and hence, female size and age.
The embryos develop lecithotrophically and hatch in about 10 days (Farke, 1979). Immediately after hatching, the juveniles dig into the sediment. Under stable conditions, juvenile Aphelochaeta marioni disperse by lateral burrowing (Farke, 1979). As there is no pelagic stage, dispersal and immigration to new areas must mainly occur during periods of erosion when animals are carried away from their habitat by water currents. At other times, recruitment must largely occur from local populations. Juvenile mortality is high (ca 10% per month) and most animals survive for less than a year (Farke, 1979). In the Wadden Sea, the majority of the cohort reached maturity and spawned at the end of their first year, although some slower developers did not spawn until the end of their second year (Farke, 1979). However, the population of Aphelochaeta marioni in Stonehouse Pool spawned for the first time at the end of the second year of life (Gibbs, 1971). There was no evidence of a major post-spawning mortality and it was suggested that individuals may survive to spawn over several years.
Most other macrofauna in the biotope breed several times in their life history (iteroparous) and are planktonic spawners producing large numbers of gametes. Dispersal potential is high. Overall recruitment is likely to be patchy and sporadic, with high spat fall occurring in areas devoid of adults, perhaps lost due to predation or storms.

Time for community to reach maturity

Cole & Hancock (1956) reported that following dredging of slipper limpets on estuarine oyster beds, it took up to 10 years for the species to reach pre-clearance population levels. The majority of the other species in the biotope are relatively short-lived and highly fecund and will probably reach mature community population levels rapidly. For example, ascidians exhibit annual episodic recruitment and are likely to achieve mature populations very quickly where suitable substrata and hydrographic conditions exist. The rapid recoverability of estuarine soft sediment infauna was reported by Hall & Harding (1997). Following suction dredging which resulted in 50% reduction in number of individuals of infauna, populations recovered to pre-dredging levels within 56 days. Therefore, assuming some colonization by Crepidula fornicata, a qualitative community would develop in a year or so, although recruitment to a mature community may take up to 10 years, taking account of the time taken for Crepidula fornicata to reach full abundance. It should be noted again that the IMX.CreAph biotope often occurs in association with declining or relict oyster beds and may be found in a transitional stage between IMX.Ost and IMX.CreAph.